This project is responsive to the Program Announcement # PA-98-011 "Impact of HIV Variation on Immunological Recognition." The aim of this proposal is to investigate cross clade HIV specific CTL responses. Stimulation of a broad cross clade reactive HIV specific CTL response is considered crucial for a successful HIV vaccine. The high degree of sequence variation is a prominent feature of HIV infection, and a major impediment to vaccine development. However, very little information is known about HIV specific CTL responses in international populations of diverse HLA types, infected with HIV clade variants. Through our collaborators, we have access to PBMC samples from HIV infected individuals in Africa, China, South America and Thailand infected with HIV-1 and HIV-2. In the first specific aim, we will determine whether cross-reactive CTL epitopes exist in diverse populations infected with different clades of HIV-1. We will measure cross reactive CTL responses with a sensitive ELISPOT assay, using both a panel of clade specific recombinant vaccinia viruses expressing HIV-1 gene products from env, gag, p01 and nef, and clade specific peptides. Bulk culture CTL responses will be generated with polyclonal and antigen specific restimulation and tested on autologous or matched B lymphoblastoid cell targets with clade specific antigens. We will correlate CTL frequency with plasma load of viral RNA for all clades and assess whether amino acid changes in optimally defined minimal epitopes can make HIV CTL epitopes more cross reactive. In the second specific aim, we will ascertain whether infection with HlV~2 stimulates CTL which cross react with HIV-1 or vice versa, and whether some individuals are protected by one clade of HIV-1 against super-infection with another.